The transportation of multiple sacks or boxes is commonly simplified by stacking them in an ordered manner on a platform, usually referred to as a pallet. This pallet, normally made of wood or plastic, can then be lifted and carried by a forklift and thereby stored in a warehouse or placed in a mobile transport, such as cargo ships or trucks. Such loading and transport arrangements can undesirably result in knocking the sacks or boxes off their pallet. In order to stabilize the load on the pallet, a film or web is commonly wrapped around the stacked items. It is desirable to pre-stretch this web past its elastic limit during the wrapping operation, in order to bind the pallet load as tightly as possible.
There are numerous prior art packaging devices available to generate the forces required to stretch and wrap a plastic web around an arbitrarily shaped pallet load. The more recent devices, such as U.S. Pat. Nos. 4,458,467 and 4,706,443, use a set of pre-stretch rollers which stretch the plastic web by a predetermined amount without putting an excessive or uneven horizontal destabilizing force on the pallet load. The tension of the prestretched web is controlled by monitoring movement of the idler rollers. The pre-stretch process also has the added benefit of smoothing out any unnecessary wrinkles present in the feedstock wrapping material.
One disadvantage of this wrapping process is the tension on the pre-stretched wrapping material does not usually remain constant while it is being applied to the pallet load. The exterior load surface is typically at varying radial distance from the load center and the load rotates in an eccentric manner because of unavoidable inexact centering on the turntable. The resulting variability in wrapping tension may produce tight and loose regions in the wrap, which can cause the wrapped load to be become unstable. There is a need to retain a predetermined web tension during wrapping so as to bind the load tightly, but the nature of the load results in a need for adjusting the web speed.
The two above mentioned prior art devices compensate for the variability in web tension by changing the speed of the roller-motor through a feedback sensor and controller system. This is accomplished in the case of 4,458,467 through the use of two idler rollers and a position transducer, which senses excessive rotational displacement of the idler assembly from its biased position. Reduction of one idler roller was accomplished by 4,706,443 but with the added consequence of a more complicated position sensor arrangement.
The use of idler rollers for feedback control compensates for the problem of variable pre-stretched web tension, but also increases the tortuosity of the path the fragile web must travel from the pre-stretch rollers to the pallet load. This increases the risk of damage to the thin pre-stretched web during the wrapping operation, which normally takes place in an industrial environment. The increase in path length for the pre-stretched web also allows the applied pre-stretch additional recovery time, which is not desirable because either the web must be over-stretched to compensate for the recovery period, or the binding strength of the wrapped web around the pallet load can be less than optimal. An additional complication is the increase in the size of the packaging device's footprint, which is undesirable in small spaces and high traffic areas. One further disadvantage of employing idler rollers to control web tension is the increased weight of the pre-stretch assembly, which must be raised and lowered throughout the wrapping operation by mechanized means.
It is an object of the present invention to obviate or mitigate the above problems.